Lipid composition of gallbladder bile, bile acid pool size and sex hormone levels were measured in 66 Pima Indians aged 10 to 22 years in order to determine the age of onset of lithogenic bile in this population noted for a high prevalence of cholesterol gallstones. Bile acid pool size is measured by isotope dilution using deuterated chenodeoxycholic acid, and gallbladder bile is obtained by duodenal siphonage after stimulation of gallbladder contraction. Changes in bile lipids and bile acid pool size were examined in relation to age, sex, hormone levels, body size, and pubertal development.